If you follow the bobistheoilguy website, the Castrol Syntec made in Germany is a group V PAO. I've yet to see it, but people seem to find it here and there and swear by it. OTherwise, Mobil 1 0w40 is ACEA-3 approved for Euro cars while normal M1 isn't. I've used M1 10w30, M1 Delvac 5w40 and find I like the D1 much more (it costs more, though).
What you do gain by going to a synthetic is better oil flow when it's cold, less suceptibility to shearing (due to heat, rpm's, engine design [some engines are harder on oil than others]) by the molecular nature of the oil, and a higher TBN (total base number, a number gauging the amount of additive in the oil). If you drive lots of short trips, this is going to reduce the life span of any oil (but if you have an initial TBN of 10 like in D1 vs. 6 like in M1 or lower like in a cheapo dino, you can drive longer theoretically). Also, I think it's the amount of silicon in the dirty oil (ie, sand from dirt in the air) that really wears your engine. The other metals merely indicate what's wearing and how much (bearings, etc.). There are lots of UOA's on the bobistheoilguy site and explanations on how to read them. My take on it is, what determines how long to run the oil is the amount of additive remaining (by the time it's down to below 2, it's shot), the amount of silicon, and the other residual metals, which should be low unless something is wrong. Also, the UOA will tell you if any coolant is leaking into the oil, etc. Other than this (getting UOA's done, say, every 3K mi), there really isn't any scientific way to know how long you should run your oil in your particular engine. I'm sure Toyota has done the same thing w/ dino oil.
